Green I mostly use on my fileserver for stuff I infrequently access, not sure how great they'd be for more demanding work. They run mostly about 5400rpm, which is fine but not amazing, and will never scale up to 7200rpm speeds even with "intelipower". They're about the most mainstream consumer drive WD offers, and you'd absolutely have to run an endurance test on it before trusting data to it. They stay darned cool, though, which should not be easily dismissed.

Red seems to be built for reliable 24/7 operation, and should be a decent option, it also uses "intelipower", so it won't be as consistently fast as Black, but probably a step up in speed from Green, and probably a little better built.

Black is 7200rpm all the time and prioritizes reads or writes on the fly to whatever it detects you're trying to do, they are performance oriented and cost more (I was confused earlier about whether black or red cost more, now I know), they're fast but do run darned hot if your case cooling isn't really well thought out.

One thing to consider about all of these drives is whether there's a difference in warranty between them.

In the end, Green is nice for storage/backups, and can be used just fine for your encodes, but isn't the best designed drive for it (maybe pick one up on the cheap to back everything up to; redundancy is good).

Red might be the best bet, be it for storage or encoding, as it is cheaper than Black, but still performance oriented and possibly better constructed than consumer drives. Red may be better warrantied than Green, too, though, classically, Black has been better warrantied than Red.